Deformation of photovoltaic modules due to their own weight

The continuous development of the photovoltaic industry and the evolution of materials and technology have brought considerable advances to products. At the beginning of the 21st century, the power of a single module was only 180 Wp, but now it can reach 700 Wp (see Figure 1). The increase in the power of photovoltaic modules has contributed to the reduction of LCOE (levelized cost of energy) of solar plants and increased the importance of photovoltaic sources among current energy sources.

Since 2019, the size of photovoltaic cells has changed, first from 156,75 mm to 158,75 mm, then to 166 mm and now they have reached 210 mm (Figure 2). With this increase in cells, the size of the modules has also gained significantly (see Table 1).

The constructive structure of photovoltaic modules is made in layers (see Figure 3). Monofacial modules receive a polymer back coating (backsheet), while bifacial modules can have a transparent polymer or glass back layer.

In the monofacial module, the glass represents about 72,3% of the total weight of the panel, while in the bifacial module, the glass (2,0 mm thick) represents about 82,96% of the total weight. Due to the self-weight of the glass, the center of the PV module may suffer sinking, as shown in the simulation result illustrated in Figure 4. The extent of this sinking depends on many factors, the most significant being the area of ​​the panel, the type (mono or bifacial), the angle and the installation method.

A study was carried out to evaluate the subsidence of the module. Firstly, we sought to evaluate the influence of the module size. The boundary conditions are shown in Table 2 and Figure 5.

Table 2: Simulation boundary conditions

In the simulation, the finite element analysis method was used to simulate the phenomenon of sinking of the center of the module, with the general Lagrange algorithm to calculate the convergence iteration. The results are shown in Table 3 and Figure 6.

Table 3: Sinking under different module sizes

Through the simulation, it is clear that the sinking of the center of the module due to its own weight is a real problem – and the larger the area, the greater the deformation suffered by the module. Likewise, the amplitude of the sinking of the center also varies with the installation angle. The module with the largest area (type C) is used to simulate the sinking at different installation angles (0°, 15°, 30° and 40°) – as shown in Figure 7.

Through simulation, it can be concluded that with a decrease in the installation angle, the phenomenon of sinking in the center of the module becomes more intense. The module installed horizontally (0° angle) suffered the greatest deformation, as shown in Table 4 and Figure 8.

Table 4. Deformations observed with different installation angles

To evaluate whether central subsidence can affect the module's energy production, the boundary conditions that suffered the greatest subsidence are adopted, that is, the type C bifacial module with the largest area, installed at 0°, is selected and the test static mechanical load is carried out in accordance with IEC 61215, as shown in Figure 8.

According to IEC 61215, the static mechanical load test sequence was conducted with 3 cycles, each carried out with a positive pressure of 5400 Pa and a negative pressure of 2400 Pa, as shown in Fig. 10.

Two samples were tested in accordance with the IEC 61215 standard and then power degradation was evaluated (with flash test) and EL (electroluminescence) images were obtained to check whether microcracks occurred in the photovoltaic cells after the tests. It is worth mentioning that the dips measured in the test were even greater than those initially assessed through finite element analysis.

From the comparison of the EL images, no microcracks were observed and no other visible flaws were found, as illustrated in Figures 11 and 12.

Power tests with flash light were carried out to compare the power degradation rate and the results were also satisfactory, as shown in Table 5.

Table 5: Power degradation after static mechanical load test

Through the results of the analysis carried out, the following conclusions could be obtained:

1. It is a fact that the modules suffer from sinking, especially in the central region, due to their own weight;
2. The central sinking of the module is mainly related to the type of module (monofacial – polymeric backsheet or bifacial – glass backsheet), the module area and its installation angle;
3. The sinking of the bifacial module with two glass sheets (front and rear) is more evident;
4. The larger the module area, the more pronounced the sinking phenomenon;
5. The smaller the installation angle, the greater the sinking phenomenon;
6. A module with the largest surface area was subjected to static mechanical load testing in accordance with IEC 61215, which provides greater sag than can be achieved with the module's own weight. After this test, no reliability problems were found in the two samples tested;
7. The deformation caused by self-weight does not affect the reliability of the photovoltaic modules, as it is lower than that observed in the static load tests of the two samples analyzed.